This invention relates generally to frequency measuring apparatus and, in particular, relates to apparatus for indicating the frequency of operation of radio apparatus capable of operation in multiple modes and of radio apparatus using multiple frequency conversion and, in greater particular, relates to apparatus for providing a direct visual indication of the carrier frequency of the signal to which a multiple mode radio apparatus is tuned.
The desirability of apparatus for providing a direct, digital, visual indication of the signal to which a radio receiver is tuned is well known in the art. Such tuning indicators have the capability of being very accurate and dispensing with the calibration, alignment, and stability problems associated with traditional dial plate tuning indicators. An example of the digital tuning indicators provided by the prior art together with a more complete discussion of the advantages of digital tuning indicators may be found in U.S. Pat. No. 3,758,853 to W. R. Dionne, et al., which issued on Sept. 11, 1973. Another digital tuning indicator of the prior art forms the subject matter of U.S. Pat. No. 3,244,983 to R. J. Ertman which issued on Apr. 5, 1966. A radio receiver capable of receiving signals on the broadcast AM and FM frequency bands and including a digital frequency indicating apparatus was manufactured by The Magnavox Company, Fort Wayne, Indiana, under the chassis number R247 and is described in its service manual 1490.
However, the digital tuning indicators of the prior art have had various disadvantages associated therewith. Many of those indicators have been excessively complex in design and construction and have been excessively costly. Many of the digital indicators of the prior art have not been sufficiently accurate to meet present day requirements. Some of the digital tuning indicators of the prior art, such as that mentioned in the above-noted patent to Ertman, have not been capable of use in multi-conversion radio apparatus. Some of the digital tuning indicators of the prior art have not been capable of use in multi-mode radio apparatus wherein the same tuning apparatus is used to tune signals having a plurality of different modes. Some of the digital tuning indicators of the prior art have relied for their operation on the generation of signals at frequencies which degraded the operation of the apparatus. As an example of the last-mentioned disadvantage, it has been found to be particularly disadvantageous in radio receiver digital tuning indicators to make use of a signal at the frequency of the signal being received as such a signal interferes with and degrades reception of the received signal.